1. Field of the Invention
The present invention relates to a radar device for scanning a predetermined angle range centered on a predetermined reference direction, and to a target position detection method of the radar device. In particular, the present invention relates to a radar device and a target detection method of the same, the radar device having: a target position detection unit which detects the position of a target within the angle range, on an XY coordinate plane having the reference direction as the Y-axis and a direction perpendicular to the reference direction as the X-axis, for each scanning; and a target position estimation unit which, on a scan in which the target position is not detected, estimates a position for detection in the scan on the basis of a first position of the target detected previously and a second position detected after the detection of the first position.
2. Description of the Related Art
Known vehicle control systems include systems for performing collision response control by way of an on-vehicle radar device that scans ahead of a cruising vehicle, to predict collision with a vehicle ahead and, thereupon, accelerate/decelerate the vehicle and/or activate safety devices.
The on-vehicle radar device in such vehicle systems transmits and receives frequency-modulated radar signals over a predetermined angle range centered on a reference direction in front of the radar device, by electronic scanning or mechanical scanning. The transmission and reception signals are analyzed by an information processing device, such as a microcomputer, provided in the on-vehicle radar device, to detect the relative speed, the relative distance and the angle of direction of the target, as well as the position of the latter within the scan plane. On the basis of these detection results, the ECU (Electronic Control Unit) of the vehicle predicts a collision and controls thereupon various actuators of the vehicle.
In scanning by an on-vehicle radar device, both the target and the radar device are moving at high speeds. As a result, the angles of the reflection surfaces of the target change from moment to moment relative to the radar signals. The level of the reception signals exhibits therefore variation, and thus high-reliability detection results cannot be always obtained. For this reason, the on-vehicle radar device determines continuity between detection results, in order to ensure the reliability of the detection results on each scan.
FIG. 1 is a diagram for explaining a continuity determination process by an on-vehicle radar device. In FIGS. 1A and 1B, a radar device 10 installed in a vehicle 1 scans the road ahead of the vehicle over a predetermined angle (α) about a reference direction F that corresponds to the vehicle forward face. In FIGS. 1A and 1B, the black circles P-1, P-2, . . . P-n represent the positions of a target detected on one scan (n denotes the scan count), while the solid arrows represent shifts in the position of the target.
The radar device 10 stores the target positions, sequentially detected through continuous scanning, in a memory of an information processing device. For each scan, the radar device 10 determines continuity on the basis of the displacement from the target position on a previous scan to the target position at the current scan, and determines that there is continuity when the displacement lies within a predetermined range. When the radar device 10 determines that there is continuity over a predetermined number of consecutive times (for instance three times), the radar device outputs the latest target position, together with information such as relative velocity and the like, to an ECU of the vehicle 1.
For instance, when in FIG. 1A the radar device 10 acknowledges continuity for three consecutive times, between the target positions P-1 and P-2, P-2 and P-3, and P-3 and P4, the radar device 10 outputs the latest target position P-4. The radar device 10 outputs thereafter target positions P-5, P-6, P-7, . . . each time that continuity is acknowledged in the same way.
In the above process, reception signals of sufficient level may fail to be received, and so the target position is not detected (the target is lost). FIG. 1B illustrates one such example, where the target is lost on the third scan. The count of continuity determination times, following on positions P-1 and P-2, is then reset, restarting at the moment that position P-4 is detected on the fourth scan. Position P-7 becomes then the earliest position for which there is acknowledged three-times continuity, which is the determination count required for output, whereupon start of the collision response control becomes delayed in proportion. This control delay increases the risk of collision when the target follows a gradually approaching trajectory, as illustrated in FIG. 1B.
Therefore, the on-vehicle radar device estimates the target position on scans where the target is lost, to maintain thereby continuity between an estimated position and a newly detected position when the target position is detected again. Doing so allows preventing delay in the output of the target position caused by a break in the continuity determination count. Japanese Unexamined Patent Application Publication No. 2004-233085 discloses an example of an on-vehicle radar device where such an estimation is carried out.
FIG. 2 is a diagram for explaining a method for target position estimation. FIG. 2 illustrates an instance in which, after acknowledging continuity between detected target positions P-1, P-2, the target is lost on the third scan, whereupon a target position P-3 is estimated for this scan. In FIG. 2, the target position is depicted on an XY plane where the Y-axis corresponds to a reference direction F ahead of the vehicle 1, the X-axis corresponds to a direction perpendicular thereto, and the origin is the position of the radar device 10.
Firstly, the radar device 10 determines a displacement Δx1 of the X coordinate and a displacement Δy1 of the Y coordinate between positions P-1 and P-2 of the target.
In the case of a forward monitoring radar, the target to be monitored is a vehicle ahead. Therefore, the Y coordinate of the target undergoes a comparatively large shift owing to, for instance, increases and decreases in relative speed. Motion of the target in the X-axis direction, however, is at most about that of a lane change, and thus displacement of the X coordinate is comparatively small. Taking Δx1 as a displacement when the X coordinate shifts substantially as a result of a lane change or the like, it is very likely that position Pd, which results from shifting the target position P-2 by Δx1 in the X-axis direction and by Δy1 in the Y-axis direction, will shift considerably beyond the actual target position in the X-axis direction.
Suppose now that after an estimation using position Pd as position P-3, the target is lost also on the next fourth scan and the position thereof is estimated (position Pd2), with position P-5 being detected then on the next fifth scan. In that case, target position Pd2 obtained through repeated estimations diverges then substantially from the detected target position P-5, and thus it is judged that there is no continuity between positions Pd2 and P-5, and the estimation of target position becomes meaningless.
In the above example, therefore, the radar device 10 for forward monitoring estimates position P-3 to be the position having an X coordinate shifted by 0.3 times the displacement Δx1, from the X coordinate of position P-2, and having a Y coordinate shifted by the displacement Δy1 from the Y coordinate of position P-2. The radar device 10 estimates position P-4 in the same way, using the displacement Δx11 of the X coordinate from positions P-2 to P-3. Doing so restricts the displacement of the estimated position in the X-axis direction, and allows hence estimating a position close to the actual target position, such that continuity with the estimated position P-4 is maintained, with high accuracy, when position P-5 is detected.
Recent years have witnessed a growing demand for vehicle control systems that can prevent so-called crossing collisions that occur when an own vehicle enters an intersection at the same time as another vehicle that cruises in a direction perpendicular to the travel direction of the own vehicle. However, there are limits to the angle range that can be detected by radar devices, either relying on mechanical scanning or electronic scanning. It is therefore difficult to detect the position of another vehicle that is cruising in a direction perpendicular to the travel direction of the own vehicle using a radar device for forward monitoring alone. Methods have been proposed therefore in which the arrangement of the on-vehicle radar device is such that the reference direction is a direction oblique to the vehicle, so as to allow monitoring ahead and to the sides of the vehicle.
As illustrated in FIG. 3, however, for instance the displacement ΔX31 of the X coordinate of the target is greater than in the case of forward monitoring, with the Y-axis being now the right-oblique forward reference direction Fd facing ahead of the vehicle, and the X-axis the direction perpendicular to the reference direction Fd. As a result, the problems below arise when the target position is lost.
FIG. 4 is a diagram for explaining a method for target position estimation when a radar device for forward monitoring is used for forward-lateral monitoring of vehicles. In the figure, the Y-axis corresponds to the right-oblique forward reference direction Fd of the vehicle. FIG. 4 illustrates an instance in which the target is lost on the third scan, after acknowledging continuity between detected target positions P-1, P-2, whereupon a target position P-3 is estimated. Estimation of position P-3 is carried out in accordance with the same method as in the case of forward monitoring illustrated in FIG. 2, by reducing the displacement of the X coordinate. In the case of forward-lateral monitoring, however, the target position changes considerably in the X-axis direction, and hence the displacement Δx2 of the X coordinate from position P-1 to position P-2 is greater than the displacement Δx1 of the X coordinate in FIG. 2. As a result, it is highly probable that the estimated position P-3 and the actual position diverge substantially in the X-axis direction. Even if the target position P-4 is detected on the immediately succeeding scan, the divergence between the estimated target position P-3 and the detected target position P-4 is substantial, so it is highly probable that continuity is not acknowledged. Output of detection results becomes delayed as a result.